The present invention relates to a process for the production of aluminum by means of electrolysis of a molten electrolyte, and also relates to an electrolytic cell for this purpose.
In order to obtain aluminum by electrolysis from aluminum oxide, this oxide is usually dissolved in a fluoride melt which consists for the most part of cryolite (Na.sub.3 AlF.sub.6). The aluminum which separates out at the cathode collects on the carbon floor of the cell under the fluoride melt, whereby the surface of the liquid aluminum forms the cathode. Anodes which, in conventional processes, are made out of amorphous carbon dip into the fluoride melt from above. At the carbon anodes, due to the electrolytic decomposition of the aluminum oxide, oxygen is formed and combines with the carbon of the anode to produce CO and CO.sub.2. The electrolysis takes place in a temperature range of about 940.degree. to 975.degree. C.
The well known principle of a conventional aluminum electrolytic reduction cell with pre-baked carbon anodes is illustrated in FIG. 1, which corresponds with the present state of the art and shows a vertical cross section longitudinally through a part of the electrolytic cell. The steel shell 12, which is lined with thermal insulation 13 of heat resistant material of low thermal conductivity and carbon 11, contains the fluoride melt 10 which constitutes the electrolyte. The aluminum 14 which precipitates out at the cathode is lying on the carbon floor 15 of the cell. The surface 16 of the liquid aluminum forms the cathode.
Embedded in the carbon lining 11, running transverse to the length of the cell, are iron cathode bars 17 which conduct the electrical direct current out of the carbon lining 11 and out of the cell at its sides.
Anodes 18 of amorphous carbon, which supply the direct current to the electrolyte, dip into the fluoride melt from above. The anodes are connected via anode rods 19 and clamps 20 to the anode beam 21.
The electrical current flows from the cathode bars 17 of one cell to the anode beam 21 of the next cell via busbars which are not shown here. It then flows from the anode beam 21 through the anode rods 19, the anodes 18, the electrolyte, the liquid aluminum 14, and the carbon lining 11 to the cathode bars 17.
The electrolyte 10 is covered with a crust 22 of solidified melt and a layer of aluminum oxide 23 on top of the crust 22. During operation of the cell there are spaces 25 between the electrolyte 10 and the solidified crust 22. There is also a crust of solidified electrolyte 24 on the side walls of the carbon lining 11. This crust of solidified electrolyte forming the lateral ledge 24 at the side walls delimits the horizontal dimensions of the bath of liquid aluminum 14 and electrolyte 10.
The distance d between the bottom face 26 of the anodes and the surface of the aluminum 16, also called the interpolar distance, can be altered by raising or lowering the anode beam 21 with the help of lifting mechanisms 27 which are mounted on the columns 28. Operating the lifting mechanism 27 raises or lowers all the anodes at the same time. The height of the anodes can also be adjusted individually by means of the clamps 20 on the anode beam 21.
As a result of the reaction with oxygen released during the electrolysis, the anodes are consumed at the bottom by about 1.5 to 2 cm per day, the amount depending on the type of cell. At the same time the surface of the liquid aluminum in the cell rises by about 1.5 to 2 cm per day. In the course of the electrolysis the electrolyte becomes depleted in aluminum oxide. At a lower concentration of 1 to 2 weight percent of aluminum oxide in the electrolyte, the so called anode effect is observed, i.e., there is a sudden increase in voltage from the normal 4-4.5 volts to a value of 30 volts and more. By then, at the latest, the crust has to be broken and new aluminum oxide added.
Under the normal mode of operation, the cell is supplied periodically with aluminum oxide, even when no anode effect occurs. In addition to that, each time the anode effect is observed, the crust must be broken and, as described above, the aluminum concentration raised by adding fresh aluminum oxide. The anode effect is, therefore, in practice always associated with cell supervision of a kind, which, in contrast to normal supervision, can be called "anode effect supervision".
The aluminum 14 which is produced as a result of the electrolysis collects on the floor 15 of the cell and is generally tapped off once daily by means of a special device, not shown.
For very many years now cell supervision has involved breaking the crust of solidified melt between the anodes and the side wall of solidified electrolyte and then adding fresh aluminum oxide. This method, which is still widely used today, is encountering increasing criticism because it is said to cause contamination of the air in the reduction plant and in the atmosphere around the plant. The pressure to have the cells sealed off and treat the waste gases has increased so much in recent years that it is now almost a compulsory measure. Proper elimination of waste gases in the plant is not possible, however, by sealing the cell if the feed of aluminum oxide has to be made along the side of the cell, between the anodes and the side wall of the cell.
In recent times, therefore, aluminum producers have been going over increasingly to central feed of the cell along its longitudinal axis. After breaking the crust, the feed of alumina takes place either locally and continuously by the point feeder system or discontinuously along the whole length of the central axis.
Years of experience with centrally fed cells have shown this method has the following disadvantages:
(a) Poor dissolution of the alumina added. PA1 (b) Pronounced formation of sludge on the cell floor. PA1 (c) Hard crust formation on the carbon floor along the long axis of the cathode. PA1 (d) It becomes difficult or even impossible to form a lateral ledge of electrolyte crust at the sides of the cell.
When electrolytic cells are centrally fed, a non-insulating sludge forms first at the point of addition of alumina and can gradually transform to an electrically insulating crust. This causes irregularities in the running of the cell and can shorten its service life, in particular because of the consumption of the side walls of the carbon anodes. This consumption of the carbon is the result of movement of the melt due to magnetic stirring effects and the stirring in turn produces pronounced, localized differences in current density.